This invention relates to a method and device for controlling the speed of a brushless DC motor provided for an electric vehicle, particularly to a method and device for controlling the speed of a brushless DC motor which processes the back EMF waves of the motor to obtain a speed feedback signal proportional to rotation speed of the motor, and which needs no speed feedback elements.
Generally, to control the speed of a brushless DC motor, it is necessary to install a DC tachometer or an encoder or the brushless DC motor as a speed feedback element. If using a DC tachometer as a speed feedback element, the feedback signal can enter into a speed compensator with an analog rotation speed command voltage. If using an encoder as a speed feedback element, the input signal, being a pulse train, must first be changed to an analog voltage through a Frequency to Voltage Converter. It then enters the speed compensator with the analog rotation speed command voltage synchronously. It is necessary to install a speed feedback element to the brushless DC motor when using these two methods to control the speed. But when being applied under some circumstances, for instance, to automatically guided vehicles and electric vehicles, though it is necessary to control the speed, a larger speed fluctuation can be tolerated. According to the above, the inventor's design provides a method and device for controlling the speed of a brushless DC motor provided for an electric vehicle. This method and device can be adapted to electric vehicles powered by batteries of 24 V, 48 VDC etc., and will decrease the production cost of electric vehicles.